Note that general sources are simplified and hence not usually acceptable references for essays. The purpose in reading them is to gain context and a broad understanding to help you understand the specialised sources.
Textbooks are often set out in student-friendly ways, with chapter overviews, subheadings, illustrations and summaries and/or practice questions at the end of chapters, and glossaries and indexes of technical terms. If you know very little, then you should read the whole chapter or section to get the big picture first. Use the contents table to find the right chapter, then read the overview and look at the illustrations and their titles to get the context before either starting at the beginning or going to the relevant section. Use the questions to check your understanding and the summary at the end to check your notes – did you and the author agree as to what was important?
These (particularly encyclopedias specific to the discipline) can give you a quick article on a topic, a definition or background information to help you understand more specialised reading. Because they are short, the articles may also contain inaccuracies, so what you read may be partly contradicted by more specific sources.
Texts written for experts contain many jargon words. You need to understand these terms to understand the text, and remember their definitions to pass the course. The easiest way to find out what these words mean is to consult a specialist dictionary or glossary (eg, a dictionary of geography). You can buy one, or look on the web for specialised glossaries. In fast-moving areas such as computing and biochemistry, print dictionaries soon become dated, so web ones are better.
Web articles can be useful introductions to a topic. Make sure they come from reputable sites (eg, .edu or .ac or .gov) as some information on the web is wrong. Searching by phrase or adding the discipline name can help target the most useful pages. Wikipedia is a useful starting point for quick overviews of topics, but generally should not be cited.
Book reviews are written by experts in the topic and can give you the background to, an overview, and critique of a difficult book that you have to read. Search for them on the web or in prestigious journals in the specific discipline.
These are written for people who already know the basics about the topic, and so give detailed information, backed by arguments and evidence. However, this means that they may be initially difficult to read. They have usually been reviewed by experts and are thus reputable sources for essays and other assignments or even PhD theses.
Monographs (ie, most books in university libraries!) are specialist books written on one topic. Besides the title and sub-title, scanning the contents and skimming the preface and/or introduction can tell you if the book is useful, and give you the outline of the author’s main argument and theoretical perspective. Go next to the chapter(s) that seems closest to the topic, then skim read to get an overview and find the most relevant parts.
These are usually scholarly articles which report and interpret original research. They assume knowledge, not only of the topic, but also of the debates which are current between academic authors in that area at the time of writing. They are designed to convince you, through argument and evidence, that the author’s conclusion is correct. You should note: the introduction showing why the work is important and what prior research found, the theoretical framework (Marxist, feminist, catastrophic evolutionist, etc), research methodology used, what was found (evidence/literature review/case studies/data/observations), and the inferences drawn from these (discussion) and their significance for the discipline knowledge (later discussion and conclusions).